This invention relates to rods for liquid-metal-cooled nuclear reactors and, more particularly, to a structure to restricting the release rate of fission gas from a rod in the case of any breach in the rod cladding.
A nuclear rod or pin contains gas which is under pressure. At the beginning of life, the gas is predominantly fill gas such as helium, any water vapor evolved from the fuel or blanket pellets, and tag gas if used. A representative gas pressure at the beginning of life is in the order of 50 psia. During operation, gas is generated by the fission of heavy atoms, either directly or as daughter products. This gas is primarily krypton or xenon or mixtures thereof and the gas pressure at the end of life may be in the range of 800 to 1500 psia, depending upon core design, fuel design and fuel location and/or management scheme.
Containment for individual rods comprises a hollow continuous cladding member having the ends thereof sealed and if a breach occurs in the continuity of the cladding member, fission gas will be released from the rod. Depending upon the range of assumptions which are applied, for example, breach location, breach size, pellet/clad gap, pressure drop factors, time in life, number of rods failing simultaneously, the consequences of fission gas release are predicted to cover a wide range of rod gas blanketing and temperature conditions. Although in actual practice, major fission gas releases have never been observed, and cladding breaches experienced to date have been benign, possible extreme accident sequences remain under study. In order to ensure maximum possible safety, it is desirable to minimize the possible deleterious effects of any breach in the cladding which might result in gas blanketing of the breached rod and operating rods adjacent thereto, which blanketing could impair the heat transfer to the liquid coolant.